The Weeks Before Denver's First Freeze Are When Mice Move In

By James Fiala, Owner — Environmental Pest Control Services, Denver CO · Published September 28, 2026 · Updated October 22, 2026

Most people find out they have mice in November. The scratching in the ceiling starts on a cold night, there are droppings behind the pantry cereal, and the dog won't stop staring at the bottom of the fridge.

By then the mice have been inside for weeks. The time to deal with it is now, before the cold arrives.

You have a deadline, and it's closer than it feels

Denver's first freeze usually comes in early to mid October. The National Weather Service puts the average first freeze at October 7. Mice don't wait for a thermometer. As nights cool through late September they start looking for somewhere warmer to spend the winter, and a house with a gap under the garage door is the easiest option on the block.

That's why repairs done in September prevent the problem, and the same repairs done in December are cleanup. The hole is the same. What's different is whether mice are already living on the inside of it.

A mouse only needs a gap about a quarter inch wide

If the mouse's head fits, the body follows. A quarter inch is smaller than most people picture, about the width of a pencil, so the gaps that matter are easy to walk past for years. On Front Range homes the usual ones are:

• Garage doors. A worn or torn bottom seal is the most common way in, and new construction isn't immune. Seals get damaged in the first few winters.
• Older foundations. Many older Denver homes sit on rubble or stone foundations with dozens of small gaps where mortar has worn away.
• Utility penetrations. Where gas lines, water lines, AC lines and cable enter the house, there's often a gap around the pipe that was never sealed properly.
• Dryer vents. A vent without a working flap is an open, heated tunnel.
• Attic and soffit vents. Mice climb better than people expect, especially up brick, stucco and downspouts.

What's feeding them

A gap lets mice in. Food is what makes them stay. Mice don't need much. They nibble small amounts all night, and a house mouse doesn't even need to drink water if it gets enough moisture from its food. A few crumbs a night is plenty. The usual sources in a Front Range home:

• Pet food. Kibble in the bag it came in, or a bowl left down overnight, is one of the most common things we find mice living near.
• Bird seed and grass seed. Bags stored in the garage or shed are a winter's supply of food for a whole family of mice.
• The pantry. Cereal, crackers, flour, rice and pasta in cardboard boxes or paper bags. Mice chew through both easily.
• Crumbs you can't see. Under the stove, behind the fridge, under the toaster, and under kids' seats and the couch.
• Trash and recycling. Bins without tight lids, and food containers that went into recycling without a rinse.
• Outside the house. Spilled bird feeders, fallen fruit, vegetable gardens, and compost piles right against the house.
• Water. A slow drip under the sink, a leaky outdoor spigot, or a pet's water bowl.

Take the food away and the mice that do get in have a much harder time settling in. Leave it out and even a well-sealed house can end up with a problem, because a mouse that slips in through an open garage door finds everything it needs and stays.

Why trapping on its own doesn't solve it

Trapping the mice already inside is necessary. But if that's all that happens, the entry point stays open, and next fall a new set of mice finds it. You end up paying for the same problem every year.

The lasting fix is two parts: remove the mice that are already in, and close the way they got in. That's why our visits include looking for entry points, not just setting traps.

Why poison isn't where we start

Store-bought poison seems like the easy answer. Here's why we don't reach for it first in a home:

• A poisoned mouse doesn't die right away. It can take days, and it often dies inside a wall. The smell that follows can last for weeks.
• Pets and kids. Loose bait gets carried, dropped and found.
• The animals that eat mice. A slow, poisoned mouse is easy prey. Owls, hawks, foxes and neighborhood cats that eat it can be poisoned too. The National Pesticide Information Center lists this "secondary poisoning" as a known risk of rodent bait.

When bait is the right tool, it goes in locked, tamper-resistant stations, usually placed outside around the home, where pets and children can't reach it. In a home with pets or kids, trapping plus a repair estimate is our starting point.

Signs they're already inside

• Small, dark, rod-shaped droppings along walls, in the pantry, or near pet food
• Scratching or light pattering in the walls, ceiling or attic at night
• Chewed cardboard, food packaging or bags of pet food and bird seed
• Shredded paper, insulation or fabric tucked into a corner or drawer
• Greasy smudges along baseboards where they run the same path over and over
• A stale, ammonia-like smell in a closed garage, cabinet or storage room

Chewed wiring is the one to take seriously. Mice gnaw constantly, and wire insulation in an attic or wall is a fire risk nobody sees until it's too late.

How we handle mice

• Active trapping. Traps go along walls and the paths mice actually use, not in the middle of the floor. Mice are wary of new objects, which is a big reason store-bought traps set out in the open catch nothing.
• Entry-point repair estimate. At the time of service our exclusion experts will inspect and give you an estimate for any repairs (exclusion) needed to keep mice from getting back in. That typically covers gaps around utility lines, garage door seals, dryer vents and foundation gaps.
• Sanitation walk-through. We walk the home with you to find what's feeding them, like pet food in bags, bird seed in the garage, or a woodpile against the house.
• Optional quarterly monitoring. For homes backing onto open space, greenbelts or empty lots, follow-up visits catch new mice before they turn into another infestation.

How to keep mice out: a prevention checklist

Outside the house

• Stand inside the closed garage in daylight. If you can see light under the door or around its edges, a mouse can get through. Replace a worn bottom seal
• Walk the foundation and look where pipes, cables and AC lines go into the house. Gaps around them need something mice can't chew, like metal mesh sealed in place. Steel wool works as a quick temporary plug but rusts over time. Spray foam on its own doesn't work; mice chew right through it
• Check that the dryer vent flap closes on its own, and that attic and crawl space vents have screens with no tears
• Put door sweeps on exterior doors, including the door from the garage into the house
• Stack firewood away from the house and off the ground
• Cut back shrubs and branches so they aren't touching the siding or roof
• Keep a strip of bare ground or gravel along the foundation instead of thick mulch or ivy, so mice have nowhere to hide right next to the wall

Inside the house

• Move pet food, bird seed and grass seed into metal or hard plastic bins with lids
• Put cereal, flour, rice and snacks into glass or hard plastic containers instead of boxes and bags
• Pick up pet bowls at night, or at least don't leave food in them
• Pull out the stove and fridge once a season and clean underneath
• Use trash cans with tight lids, and take kitchen trash out every night
• Fix dripping faucets and leaks under sinks
• Clear clutter in the garage, basement and storage rooms. Piles of boxes and old clothes are where mice nest

None of this is a guarantee on its own. But a house with its gaps closed and its food put away is a house mice pass by for the one next door.

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